This invention relates to a circuit for eliminating in-phase voltage of a Hall element.
Hall elements, which are used for magnetic sensors such as Gauss meters and the like, convert magnetic variation into electrical signal variation. As well known, the Hall element includes a semiconductor plate such as GaAs, InSb, InAs, Ge or Si, having a pair of control current input terminals for supplying a control current to the semiconductor plate and a pair of output terminals. When a magentic field is applied to the Hall element in a direction perpendicular to the control current direction while the control signal is supplied to the Hall element, an output signal voltage (a Hall output voltage) corresponding to the magnitude of the magnetic field is produced at the output terminals.
This output signal, however, includes a certain D.C. voltage besides the Hall output voltage. This D.C. voltage is called "in-phase voltage" and is produced on account of a voltage drop between the control current input terminal and the output terminals of the Hall element. A Hall element can be conceptualized as a network of four resistive elements having a common connection. When the connection is at a voltage other than ground, the in-phase voltage results. Accordingly, the in-phase voltage is generated independent of the application of the magnetic field.
Generally, this in-phase voltage is much larger than the Hall output voltage. For example, a GaAs Hall element having an integrating sensitivity of 20 mV/Ma.KG and an internal resistance 1 k.OMEGA. produces a Hall output voltage of 20 mV, when a control current 1 mA and a magnetic field having a magnetic flux density 1 KG are applied to the Hall element. However, the Hall element also produces an in-phase voltage of 500 mV. The Hall output voltage biased by the in-phase voltage cannot be amplified sufficiently by an amplifier because the amplification ability of the amplifier cannot be utilized efficiently for amplifying the Hall output voltage. Accordingly, the in-phase voltage should be eliminated.
One known method for eliminating the in-phase voltage of the Hall element is by employing a differential amplifier for canceling the in-phase voltages produced at the pair of output terminals as described in U.S. Pat. Nos. 4,199,696 and 4,200,814.
This differential amplifier, however, should have a high common-mode rejection ratio (CMRR) in order to cancel the in-phase voltages accurately. To obtain a high CMRR the ratio of resistances used in the differential amplifier should be adjusted accurately by employing highly accurate resistances or variable resistances. Accordingly, such an in-phase voltage eliminating circuit using a differential amplifier has a complicated structure and costs too much.